As described in the incorporated-by-reference U.S. Pat. No. 7,762,074, gas turbine engines having can-annular burner-type combustors are known, wherein individual cans feed hot combustion gas into respective individual portions of an arc of a turbine section inlet. Each can typically includes a basket, which circumscribes and retains a main burner having a plurality of premixers, which are also commonly referred to as preswirlers, disposed in an annular ring around a central pilot burner, for premixing fuel and air. The premixers receive respective portions of a flow of compressed air from the engine's compressor section, along with respective portions of a fuel flow. The respective portions of the fuel flow are discharged by fuel outlets disposed within the premixers to form a fuel-air mixture that moves through the combustor basket in a thru-flow direction for combustion in the downstream combustion zone.
The combustor basket's thru-flow airflow profile is evaluated along an air inlet plane, which is oriented perpendicular to the thru-flow direction, upstream of the premixers. For example, in a cylindrical or frusto-conical profile combustor basket, the air inlet plane is oriented perpendicular to the basket central axis upstream of the premixers. An airflow reversal region is oriented in the combustor basket on the upstream of the air inlet plane and the premixers relative to the thru-flow direction. The airflow reversal region regulates the thru-flow airflow pressure by allowing regulated, circumferential entry or intake of compressor air upstream of the air inlet plane, from outside the basket. In this known type of combustor, the compressed air flows in a reverse flow direction (relative to the thru-flow direction) around the exterior of the basket. The compressor-supplied airflow entry into the airflow reversal region is sometimes regulated by a flow conditioner that circumscribes the combustor basket airflow reversal region. The flow conditioner has a pattern of perforations, the cross sectional area of which regulates compressor airflow entering the basket. Reverse airflow into the combustor basket and thru-flow specifications are established for a gas turbine engine. Ideally, the airflow profile of the fuel-air thru-flow is constant across the entire combustor basket air inlet plane. Therefore, in the past, perforation patterns of flow conditioners have been symmetrical along the flow conditioner's circumferential surface to facilitate uniform reverse airflow from the compressor into the annular combustor basket, which was complimentary to the presumed ideal fuel-air mixture thru-flow uniform flow pattern across the air inlet plane.
While ideally, fuel-air thru-flow across a combustor basket's air inlet plane should be uniform; in reality, they experience non-uniform thru-flow. The incorporated-by-reference U.S. Pat. No. 7,762,074, describes experiments, which determined that airflow rates through respective premixers of the main burner of an individual can might vary by as much as 7.5% from an average flow rate among the premixers. The same patent states that such a variation may create temperature differentials of +/−75 degrees centigrade among the premixers when operating the gas turbine is operating at base load. These temperature differentials may result in more oxides of nitrogen (NOx) production by the relatively hotter areas of the burner associated with premixers receiving a relatively lower than average airflow and more carbon monoxide (CO) production by the relatively cooler areas of the burner associated with premixers receiving relatively more than average airflow. The incorporated-by-reference U.S. Pat. No. 7,762,074, describes a uniform, symmetrical perforation pattern of slots formed in the combustor basket flow conditioner circumference that mitigates airflow differences among premixers in a combustor can, resulting in improved combustion characteristics, such as reduced emissions.